The present invention relates to a solid multicomponent membrane which is particularly suited as dense oxygen separation membrane in applications with high driving forces for oxygen transport.
Inorganic membranes show promise for use in commercial processes for separating oxygen from an oxygen containing gaseous mixture. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle plants. This technology encompasses two different kinds of membrane materials; oxygen ion conductors and mixed oxygen ion and electronic conductors. In both cases the oxygen ion transport is by oxygen ion vacancies or interstitial oxygen in the membrane material. In the case of mixed conductors electrons are also transported in the membrane material.
Membranes formed from mixed conducting oxides can be used to selectively separate oxygen from an oxygen containing gaseous mixture at elevated temperatures. Oxygen transport occurs when a difference in the chemical potential of oxygen (xcex94logpO2) exists across the membrane. On the high oxygen partial pressure side of the membrane, molecular oxygen dissociates into oxygen ions which migrate to the low oxygen partial pressure side of the membrane and recombine there to form oxygen molecules. Electrons migrate through the membrane in the opposite direction to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by three processes; (I) the rate of oxygen exchange at the high oxygen partial pressure surface of the membrane, (II) the oxygen diffusion rate within the membrane and (III) the rate of oxygen exchange on the low oxygen partial pressure surface of the membrane. If the rate of oxygen permeation is controlled by the oxygen diffusion rate, the oxygen permeability is known to be inversely proportional to the membrane thickness (Fick""s law). If the membrane thickness is decreased below a certain critical membrane thickness which depends on temperature and other process parameters, surface oxygen exchange on one or both membrane surfaces will become oxygen permeation rate limiting. The rate of oxygen permeation is then independent of the membrane thickness.
During recent years the use of dense mixed conducting membranes in various processes has been described. Examples are oxygen production described in European Patent Application No. 95100243.5 (EP-A-663230), U.S. Pat. No. 5,240,480, U.S. Pat. No. 5,447,555, U.S. Pat. No. 5,516,359 and U.S. Pat. No. 5,108,465, partial oxidation of hydrocarbons described in U.S. Pat. No. 5,714,091 and European Patent Application No. 90134083.8 (EP-A438902), production of synthesis gas described in U.S. Pat. No. 5,356,728 and enrichment of a sweep gas for fossil energy conversion with economical CO2 abatement as described in PCT/NO97/00170, PCT/NO97/00171 and PCT/NO97/00172.
For the application of MCM (Mixed Conducting Membrane) technology, the membrane material must fulfil certain requirements in addition to being a good mixed conductor. These fall into three categories; thermodynamic stability under static conditions, thermodynamic stability under dynamic conditions, and mechanical stability. The membrane material must be thermodynamically stable under any static condition within the appropriate temperature and oxygen partial pressure range. Furthermore, the membrane material must be stable against reaction with the additional components in the gaseous phase (e.g. CO2, H2O, NOx, SOx), and any solid phase in contact with it (e.g., seals and support material). This calls for different materials for different applications.
A membrane material that fulfils all the stability requirements under static conditions, may still be unstable when it is placed in a potential gradient. Any multi-component material kept in a potential gradient, e.g. oxygen partial pressure gradient or electrical potential gradient, will be subjected to driving forces acting to demix or decompose the material. These phenomena are called kinetic demixing and kinetic decomposition and are well described in the literature (e.g., Schmalzried, H. and Laqua, W., Oxidation of Metals 15 (1981) 339).
Kinetic demixing acts to gradually change the cationic composition of the membrane along the axis parallel to the applied potential. This phenomenon will always occur in materials where a mixture of cations are present on the same sublattice. Kinetic demixing may or may not reduce the performance and lifetime of the membrane.
Kinetic decomposition implies a total breakdown of the compound or compounds comprising the membrane, and results in the appearance of decomposition compounds on the membrane surface. This phenomenon occurs in all multicomponent materials when placed in a potential gradient exceeding a certain critical magnitude. A membrane kept in an oxygen partial pressure gradient large enough for kinetic decomposition to take place, will have its performance and lifetime reduced. Those skilled in the art recognize the phenomenon of kinetic decomposition as one of the major critical parameters in developing durable membranes, particularly for processes involving large potential gradients across the membrane.
Furthermore, when the membrane is placed in an oxygen chemical potential gradient and it responds by establishing a gradient in the concentration of oxygen vacancies or interstitials parallel to the direction of the applied potential, the membrane experiences mechanical stress with the strain plane perpendicular to the direction of the applied potential gradient. This mechanical stress is caused by a phenomenon referred to as chemical expansion, which can be defined as the dependency of the unit cell volume of the nonstoichiometric oxide on the oxygen stoichiometry. When the chemical expansion exceeds a critical limit, and gives rise to mechanical stress exceeding a critical limit governed by the membrane package design, a mechanical failure of the membrane package may result Those skilled in the art recognize the phenomenon of chemical expansion as one of the major critical parameters in developing durable membrane packages.
Two prior art processes can be put forward as particularly relevant to the present invention: the production of synthesis gas in which an oxygen containing gas is fed to the first side of a membrane, whereby pure oxygen is transported through the membrane, and the so produced oxygen partially oxidizes a hydrocarbon containing gas supplied to the second side of the membrane; and fossil energy conversion with economical CO2 abatement (e.g. PCT/NO97/00172) where an oxygen containing gas is fed to the first side of a membrane, whereby pure oxygen is transported through the membrane, and the produced oxygen oxidizes a hydrocarbon containing gas supplied to the second side of the membrane.
The process conditions of the relevant process define the environs of the membrane and play a determining role in the selection of membrane material. Examples of typical process parameters for the two said processes are given in Tables 1 and 2, respectively. Both processes are characterized by a logpO2 gradient across the membrane of well above 10 decades. Furthermore, both processes call for membrane materials that have a high stability against reaction with CO2 under reducing conditions, as the CO2 pressure is well above 1 bar.
During recent years dense mixed conducting membranes have been described.
U.S. Pat. No. 5,306,411 discloses a solid, gas-impervious, electron-conductive, oxygen ion-conductive, single-phase membrane for use in an electrochemical reactor, said membrane being formed from a perovskite represented by the formula:
AsAxe2x80x2tBuBxe2x80x2vBxe2x80x3wOx
wherein A represents a lanthanide, Y, or mixture thereof; Axe2x80x2 represents an alkaline earth metal or mixture thereof; B represents Fe; Bxe2x80x2 represents Cr, Ti, or mixture thereof; and Bxe2x80x3 represents Mn, Co, V, Ni, Cu, or mixture thereof and s, t, u, v, w, and x each represent a number such that:
s/t equals from about 0.01 to about 100;
u equals from about 0.01 to about 1;
v equals from about 0.01 to 1;
w equals from zero to about 1;
x equals a number that satisfies the valences of the A, Axe2x80x2, B, Bxe2x80x2 and Bxe2x80x3 in the formula; and
0.9 less than (s+t)/(u+v+w) less than 1.1.
The examples focusing on Axe2x80x2 representing Sr and Bxe2x80x2 representing Cr.
U.S. Pat. No. 5,712,220 describes compositions capable of operating under high carbon dioxide partial pressures for use in solid-state oxygen producing devices represented by the formula LnxAxe2x80x2xAxe2x80x3xByBxe2x80x2yBxe2x80x3yO3xe2x88x92x, wherein Ln is an element selected from the f block lanthanides, Axe2x80x2 is selected from Group 2, Axe2x80x3 is selected from Groups 1, 2 and 3 and the f block lanthanides, and B, Bxe2x80x2, Bxe2x80x3 are independently selected from the d block transition metals, excluding titanium and chromium, wherein 0 less than =x less than 1, 0 less than xxe2x80x2 less than 1, 0 less than =xxe2x80x3 less than 1, 0 less than y less than 1.1, 0 less than yxe2x80x2 less than 1.1, 0 less than =yxe2x80x3 less than 1.1, x+xxe2x80x2+xxe2x80x3=1.0, 1.1 greater than y+yxe2x80x2+yxe2x80x3 greater than 1.0 and z is a number which renders the compound charge neutral wherein such elements are represented according to the Periodic Table of the Elements adopted by IUPAC. The examples focusing on Axe2x80x2 representing Sr or Ba, B representing Co, Bxe2x80x2 representing Fe, and Bxe2x80x3 representing Cu.
WO97/41060 describes a solid state membrane for use in a catalytic membrane reactor wherein said membrane is fabricated from a mixed metal oxide material having a brownmillerite structure and having the general stoichiometry A2xe2x88x92xAxe2x80x2xB2xe2x88x92yBxe2x80x2yO5+z, where A is an alkaline earth metal ion or mixture of alkaline earth metal ions; Axe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group consisting of the lanthanide series or is yttrium; B is a metal ion or mixture of metal ions wherein the metal is selected from the group consisting of 3d transition metals, and the group 13 metals; Bxe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group consisting of the 3d transition metals, the group 13 metals, the lanthanides and yttrium; x is a number greater than 0 and less than 2, y is a number greater than 0 and less than or equal to 2, and z is a number greater than zero and less than one that renders the compound charge neutral. The examples focus on the most preferred combination of elements given by A representing Sr, Axe2x80x2 representing La, B representing Ga, and Bxe2x80x2 representing Fe.
U.S. Pat. No. 5,306,411, U.S. Pat. No. 5,712,220, and WO97/41060 each encompass wide ranges of membrane compositions. It is known to those skilled in the art that a great number of compositions encompassed by the claims of U.S. Pat. No. 5,306,411 and U.S. Pat. No. 5,712,220 are inherently unstable as perovskites and that a great number of compositions encompassed by WO97/41060 are inherently unstable as brownmillerites under all conditions relevant to membrane processes. Furthermore, a large number of the compositions encompassed by U.S. Pat. No. 5,306,411, U.S. Pat. No. 5,712,220, and WO97/41060 are characterised by low or zero oxygen flux under all conditions relevant to membrane processes.
The main object of the present invention was to arrive at an improved membrane showing good stability against reaction with carbon dioxide and against reduction of oxide components to metal.
Another object of the present invention was to arrive at an improved membrane showing stability against kinetic decomposition and resistance to mechanical failure due to chemical expansion stresses.
The inventors found that a certain class of multicomponent metallic oxides are particularly suited as membrane materials in processes in which the membrane is subjected to a large potential gradient, e.g. oxygen partial pressure difference of 6-7 orders of magnitude or more across the membrane. These compositions overcome problems associated with kinetic decomposition. Additionally, due to their low chemical expansion and high stability against carbon dioxide and water, these materials are particularly suited as membranes for the production of syngas and for fossil energy conversion with economical CO2 abatement.
The compositions according to the present invention are based on the so called perovskite structure, named after the mineral perovskite, CaTiO3, but the cation stoichiometry is different from the ideal perovskite, and it is this difference that gives the compositions according to the present invention superior stability in a potential gradient. Furthermore, the process conditions associated with the production of syngas or fossil energy conversion with economical CO2 abatement limit the selection of elements of which the perovskite membrane can consist.
A material possessing the perovskite structure can in its most general form be written AvBwO3xe2x88x92d, where A and B each represent any combination and number of elements provided that the ionic radii of the elements, as defined and tabulated by Shannon (Acta Cryst. (1976) A32, 751), satisfy the requirement that the number t defined by   t  =                    r        A            +              r        O                            2            ⁢              (                              r            B                    +                      r            O                          )            
is not less than about 0.85 and not greater than about 1.10, and preferably t is not less than about 0.95 and not greater than about 1.05, where rA and rB represent the weighted average ionic radius of the A-elements and the B-elements, respectively, rO represents the ionic radius of the oxygen ion; and v, w, and d each represent numbers such that 0.9 less than v less than 1.05, 0.9 less than w less than 1.05, and d is not less than zero and not greater than about 0.8, and preferably 0.95 less than v less than 1.03 and 0.95 less than w less than 1.03.
The perovskite membrane for use in said processes must contain at least one element (I) whose valence is substantially mixed under said process conditions, and (II) with the additional requirement that the oxide of said element, or of any additional element of which the membrane is composed, does not reduce to a metal under any condition encompassed by said process conditions. This requirement points to the group of 3d transition metals, but with the limitation expressed by part (I) of the requirement excluding Sc, Ti, V, Cr, and Zn as the mixed valence element, and part (II) excluding Co, Ni, and Cu. Therefore, only Fe and Mn satisfy part (I) and part (II) of said requirement, and, hence, the membrane must contain Fe or Mn or mixture thereof. The membrane can not contain Co, Ni, or Cu. Therefore, the preferred compositions of U.S. Pat. No. 5,712,220, referenced in the xe2x80x9cBackground of the inventionxe2x80x9d, can not be used as membranes in said processes. Said preferred compositions of U.S. Pat. No. 5,712,220 are expected to decompose under the conditions fo the said two processes, resulting in decreasingly poor oxygen permeation and eventually to cracking and complete breakdown of the membrane.
Said perovskite membrane containing Fe or Mn or a mixture thereof as the B cation(s) or as constituents of the mixture of B cations, must contain A cations stable as di- or tri-valent oxides of suitable ionic radii relative to the ionic radii of Fe and Mn according to said requirement for the value of t. This limitation in combination with the exclusion of radioactive elements effectively excludes all elements according to the Periodic Table of the Elements adopted by IUPAC, except Ca, Sr, Ba, and La.
Among the oxides of Ca, Sr, Ba, and La, the oxides of Sr and Ba are not sufficiently stable with respect to formation of carbonates, SrCO3 and BaCO3, to be used in said processes for which typical process parameters were given in Tables 1 and 2. The stability of the oxides of Ca, Sr, Ba, and La relative to the corresponding carbonates are shown in FIG. 1. Hence, for said processes, only La and Ca can be used as A-cations in the perovskite of which the membrane consists. The exclusion of Sr and Ba as constituents of the membrane, excludes the use of the preferred compositions of U.S. Pat. No. 5,712,220, U.S. Pat. No. 5,306,411, and WO97/41060, referenced in the xe2x80x9cBackground of the inventionxe2x80x9d, as membranes in said processes. Said preferred compositions of U.S. Pat. No. 5,712,220, U.S. Pat. No. 5,306,411, and WO97/41060, all containing Sr or Ba, are expected to react with CO2 and decompose under the formation of SrCO3 and BaCO3 under the conditions of said two processes, resulting in decreasingly poor oxygen permeation and eventually cracking and complete breakdown of the membrane.
In addition to containing the mixed valence elements Fe or Mn, or a mixture of Mn and Fe, the perovskite for use as a membrane in said processes can also contain one or more fixed valence elements as B cations; fixed valence meaning here that the particular ion has substantially the same valency at any spatial point in the membrane and at any time for the relevant process. The presence of such fixed valence elements may be needed in order to Increase the stability of the perovskite, to decrease the chemical expansion, to prevent ordering, or to enhance the performance of the perovskite as a membrane material in any other manner. The ions of the fixed valence elements must be of suitable ionic radii relative to the other B cations and A cations, according to said requirement for the value of t defined above. This limitation excludes all other elements than Ti, Cr, Al, Ga, Ge and Be. Furthermore, due to high vapor pressures of Ge containing species and low melting temperatures, Ge has to be excluded. Be is excluded on grounds of toxicity and high vapor pressure of the hydrate of beryllium. Of the remaining elements Al and Ga are expected to have similar effect as constituents in the perovskite, but the ionic radius of the Al ion is more favorable than of the Ga ion. Furthermore, the cost of Al is considerably lower than the cost of Ga. Hence, Ga can be excluded on the grounds of Al being a better choice. Under oxidizing conditions, the vapor pressure of CrO3(g) above chromium containing perovskites is high, and Cr is preferably avoided. Therefore, as a fixed valence B cation, only Ti and Al will be considered further.
The exclusion of Ga and Cr excludes the use of the preferred compositions of U.S. Pat. No. 5,306,411 and WO97/41060, referenced in the xe2x80x9cBackground of the inventionxe2x80x9d, as membranes in said processes. Said preferred compositions of U.S. Pat. No. 5,306,411 containing Cr, are expected to become depleted in Cr as CrO3(g) evaporates from the surface of the membrane under the conditions of said two processes, resulting in decomposition of the membrane material and the formation of new compounds, which yields decreasingly poor oxygen permeation and eventually cracking and complete breakdown of the membrane.
According to said requirements, limitations, and exclusions treated above, the membrane material possessing the perovskite structure for use in said processes, must have a composition represented by the formula:
(La1xe2x88x92xCax)v(B1xe2x88x92yBxe2x80x2y)wO3xe2x88x92d
wherein B represents Fe or Mn or mixture thereof; Bxe2x80x2 represents Ti or Al or mixture thereof; and x, y, v, w, and d each represent a number such that 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0.9xe2x89xa6vxe2x89xa61, 0.9xe2x89xa6wxe2x89xa61, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8, and preferably 0.95xe2x89xa6vxe2x89xa61 and 0.95xe2x89xa6wxe2x89xa61.
Compositions containing no Ti or Al, i.e. y=0, are characterized by too high chemical expansion, as exemplified by the present Example 18, and can not be used as membranes in said processes. The chemical expansion is higher for compositions containing Mn than for compositions containing Fe.
Compositions containing Ti, Al, or Ti and Al, i.e. Bxe2x80x2 represents Ti, Al, or a mixture of Ti and Al, and y greater than 0, are characterized by an improved (lower) chemical expansion as compared with compositions containing no Ti and no Al, i.e. y=0, as exemplified by a comparison of the present examples 17 and 18. The compositions of Example 17 with B representing Fe display chemical expansion characteristics that are acceptable for a membrane material in said processes.
Compositions containing Ti and Al, i.e. Bxe2x80x2 represents a mixture of Ti and Al, and y greater than 0, are characterized by a further improvement (reducton) in the chemical expansion compared with compositions where Bxe2x80x2 represents Ti and y greater than 0, as exemplified by a comparison of the present Examples 17 and 21. The composition of the present Example 21 with B representing Fe displays excellent chemical expansion characteristics for a membrane material in said processes.
Although compositions containing Mn and Ti, Al, or Ti and Al, i.e. Bxe2x80x2 represents Ti, Al, or mixture of Ti and Al, and y greater than 0 and B represents Mn, are characterized by an improved (lower) chemical expansion as compared with compositions containing no Ti or Al, i.e. y=0, the improvement is not large enough to render these compositions acceptable as membrane materials in said processes. The membrane can, therefore, not contain substantial amounts of Mn. The present Example 20 exemplifies the high chemical expansion of Mn containing materials.
Following the discussion and further limitations hitherto, the membrane material possessing the perovskite structure for use in said processes, must have a composition represented by the formula:
(La1xe2x88x92xCax)v(Fe1xe2x88x92yxe2x88x92yxe2x80x2TiyAyxe2x80x2)wO3xe2x88x92d
wherein x, y, yxe2x80x2, v, w, and d each represent a number such that 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6yxe2x80x2 less than 1, 0 less than (y+yxe2x80x2) less than 1, 0.9xe2x89xa6vxe2x89xa61, 0.9xe2x89xa6wxe2x89xa61, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8, and preferably 0.95xe2x89xa6vxe2x89xa61 and 0.95xe2x89xa6wxe2x89xa61.
Stoichiometric perovskite compositions represented by said formula, i.e. v=w=1, are kinetically unstable when subjected to large gradients (6-7 orders of magnitude or more) in the oxygen partial pressure. The kinetic decomposition that occurs in these materials gives rise to the formation of decomposition products on at least one of the membrane surfaces and a decrease in the oxygen flux with time. Such kinetic decomposition in the stoichiometric perovskite materials is exemplified by the present examples 12 and 15 and FIGS. 4, 8, 9, and 10. Kinetic decomposition becomes more pronounced when w greater than v. Therefore, stoichiometric perovskites (v=w), or perovskites with A-site deficiency (w greater than y) represented by said formula can not be used as membranes in said processes.
The exclusion of stoichiometric and A-site deficient perovskites, excludes the use of the compositions of U.S. Pat. No. 5,712,220 and WO97/41060, and excludes the use of the preferred compositions of U.S. Pat. No. 5,306,411 referenced in the xe2x80x9cBackground of the inventionxe2x80x9d, as membranes in said processes. Said compositions of U.S. Pat. No. 5,712,220, U.S. Pat. No. 5,306,411, and WO97/41060, are expected to decompose in the large oxygen partial pressure gradient of said two processes, resulting in decreasingly poor oxygen permeation and eventually to cracking and complete breakdown of the membrane.
Compositions represented by said formula, and where the numbers v and w are selected such that v=1 and 0.95xe2x89xa6w less than 1, however, are stable with respect to kinetic decomposition even in oxygen partial pressure gradients of well above 10 decades. Under certain additional requirements regarding the values of x and y of the enumerated formula, said compositions are characterized by stable oxygen flux not decreasing with time, and single phase unchanged membrane surfaces and interior. Examples of the performance of such compositions are presented in the present Examples 11, 13, and 14 and FIGS. 3, 5, 6, and 7.
Following the further limitations pointed out in the discussion hitherto, the membrane material possessing the perovskite structure for use in said processes, must have a composition represented by the formula
La1xe2x88x92xCax(Fe1xe2x88x92yxe2x88x92yxe2x80x2TiyAlyxe2x80x2)wO3xe2x88x92d
wherein x, y, yxe2x80x2, w, and d each represent a number such that 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6yxe2x80x2 less than 1, 0 less than (y+yxe2x80x2) less than 1, yxe2x89xa6x, 0.95 less than w less than 1, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8.
The compositions represented by said formula can alternatively be represented by mixtures of y number of moles of CaTiwO3xe2x88x92dxe2x80x2(CT), (xxe2x88x92y) number of moles of CaFewO3xe2x88x92dxe2x80x3 (CF), (1xe2x88x92xxe2x88x92y) number of moles of LaFewO3xe2x88x92dxe2x80x2xe2x80x3 (LF), and yxe2x80x2 number of moles of LaAl.wO3xe2x88x92dxe2x80x3xe2x80x3 (LA), with respective mole fractions given by XCT=y, XCF=xxe2x88x92y, XLF=1xe2x88x92xxe2x88x92yxe2x80x2, and XLA=yxe2x80x2. Graphically, said mixtures can be represented within a ternary phase diagram as shown in the present FIG. 14.
Compositions represented by said formula, and where the numbers x, y, and yxe2x80x2 are selected such that (y+yxe2x80x2) less than 0.1 and (xxe2x88x92y)xe2x89xa60.3 are characterized by having high chemical expansion, and membranes of these compositions can probably not be used in said processes. Examples of the high chemical expansion of these materials are presented in the present Examples 18 and 19.
Compositions represented by said formula, and where the numbers x and y are selected such that (xxe2x88x92y) less than 0.05 are characterized by having low vacancy concentrations (d), which yield low oxygen flux rates, and membranes of these compositions can probably not be used in said processes. An example of the low oxygen flux of these compositions is provided in the present Example 23.
Compositions represented by said formula, and where the numbers x, y, and yxe2x80x2 are selected such that either (y+yxe2x80x2) greater than 0.8, or (1xe2x88x92xxe2x88x92yxe2x80x2) less than 0.05 and (xxe2x88x92y)xe2x89xa60.3, are characterized by having low electronic conductivity, which yield low oxygen flux rates, and membranes of these compositions can probably not be used in said processes. An example of the low oxygen flux of these compositions is provided in the present Example 24.
Compositions represented by said formula, and where the numbers x and y are selected such that (xxe2x88x92y) greater than 0.3, are not simple perovskites at conditions representative of said processes. The cations and oxygen vacancies of these compositions become ordered, during which ordering process the flux rates decrease to eventually reach too low permeation rates to be used as membranes in said processes. An example of the low oxygen flux of these compositions is provided in the present Example 25.
Compositions represented by said formula, and where the numbers x and y are selected such that 0.1xe2x89xa6(y+yxe2x80x2)xe2x89xa60.8, 0.15xe2x89xa6(x+yxe2x80x2)xe2x89xa60.95, and 0.05xe2x89xa6(xxe2x88x92y)xe2x89xa60.3 are characterized by having properties acceptable for use as membranes in said processes. These properties include low and acceptable chemical expansion below 0.1% (Examples 17 and 21), sufficiently high vacancy concentration to yield sufficient flux rates (Example 11), sufficiently high electronic conductivity to yield sufficient flux rates (Example 11), minor (acceptable) or no ordering of cations and oxygen vacancies (Examples 11, 13 and 14).
Compositions represented by said formula and where the numbers x and y are selected such that 0.1xe2x89xa6(y+yxe2x80x2)xe2x89xa60.8, 0.15xe2x89xa6(x+yxe2x80x2)xe2x89xa60.95, 0.05xe2x89xa6(xxe2x88x92y)xe2x89xa60.3, and yxe2x80x2 greater than 0, are characterized by a further reduction in the chemical expansion (Example 21).
Thus, the membrane material according to the present invention for use in said processes has a composition represented by the formula:
La1xe2x88x92xCax(Fe1xe2x88x92yxe2x88x92yxe2x80x2TiyAlyxe2x80x2)wO3xe2x88x92d
wherein x, y, yxe2x80x2, w, and d each represent a number such that 0.1xe2x89xa6(y+yxe2x80x2)xe2x89xa60.8, 0.15xe2x89xa6(x+yxe2x80x2)xe2x89xa60.95, 0.05xe2x89xa6(xxe2x88x92y)xe2x89xa60.3, 0.95 less than w less than 1, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8.
Particularly suitable compositions according to the present invention are represented by said general formula wherein x, y, yxe2x80x2, w, and d each represent a number such that 0.15 less than (y+yxe2x80x2) less than 0.75, 0.2 less than (x+yxe2x80x2) less than 0.9, 0.05 less than (xxe2x88x92y) less than 0.15, 0.95 less than w less than 1, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8.
Representative compositions include La0.65Ca0.35Fe0.63Ti0.24Al0.10O3xe2x88x92d, La0.45Ca0.55Fe0.48Ti0.39Al0.10O3xe2x88x92d, La0.4Ca0.6Fe0.49Ti0.43Al0.05O3xe2x88x92d, La0.58Ca0.42Fe0.63Ti0.31Al0.03O3xe2x88x92d, La0.4Ca0.6Fe0.485Ti0.485O3xe2x88x92d, La0.55Ca0.45Fe0.63Ti0.34O3xe2x88x92d, La0.68Ca0.32Fe0.73Ti0.25O3xe2x88x92d and La0.22Ca0.78Fe0.34Ti0.62O3xe2x88x92d.
The improvements afforded by the applicants"" invention can be best appreciated by a comparison of properties, such as structure, performance during oxygen permeation, phase composition after permeation etc., of the claimed non-stoichiometric compositions with the prior art stoichiometric compositions.